Been running this for about three months now. I've never had a spammer on my forum and I don't use CAPTCHA, so I got curious and checked the logs.
My forum is private. There are five legitimate user registrations total. The rest were created manually through the admin interface.
There are 1,600 entries in the FoolBot log. All blocked registrations detected from spam bots. This addon is the real deal and the best spam protection I've ever seen on any web app. If you're using the free version, seriously considering buying this to support the dev.